Conventional wireless communication transmitters typically employ a quadrature modulator (also referred to as an “I/Q” modulator) to modulate two orthogonal baseband data streams—an in-phase data stream and a quadrature-phase data stream—onto a radio frequency (RF) carrier. FIG. 1 is a block diagram of a prior art quadrature modulator 10. The quadrature modulator 1 comprises an I-channel mixer 10, a Q-channel mixer 12, a local oscillator (LO) 14, a phase shifter 16 and a summer 17. The I-channel mixer 10 is configured to receive the in-phase data stream and a radio frequency (RF) carrier signal from the LO 14. At the same time, the Q-channel mixer 12 is configured to receive the quadrature-phase data stream and a ninety-degree phase shifted version of the carrier signal, by operation of the ninety-degree phase shifter 16. The I- and Q-channel mixers 10 and 12 upconvert both the in-phase and quadrature-phase data streams to the frequency of the RF carrier. The summer 17 combines the upconverted in-phase and quadrature-phase signals and feeds the sum to an input of an RF power amplifier (RFPA) 18. The RFPA 18 amplifies the upconverted sum and feeds the upconverted sum to an antenna 19, which radiates the modulated RF carrier for reception by an RF receiver.
A significant drawback of the I/Q modulator is that it is not very power efficient, especially when used to condition and transmit non-constant-envelope signals such as EDGE (Enhanced Data Rates for GSM (Global System for Mobile Communications) Evolution) and W-CDMA (Wideband Code Division Multiple Access). To minimize distortion of the signal peaks when conditioning and transmitting such signals, the drive levels to the RFPA must be reduced to prevent signal clipping, and the RFPA must be configured to amplify in a linear mode of operation. Unfortunately, linear power amplifiers are not particularly efficient.
A polar modulator is a type of modulator that avoids the linearity requirement of the RFPA and, because so, is considerably more efficient than the conventional I/Q modulator. FIG. 2 is a block diagram illustrating the principle components of a typical prior art polar modulator 2. The polar modulator 2 comprises a rectangular-to-polar converter 20; an amplitude modulator 22 configured within a magnitude path of the modulator 2; and a phase modulator 24 and voltage controlled oscillator (VCO) 26 configured within a phase path of the modulator 2.
The rectangular-to-polar converter 20 converts I and Q baseband data streams into separate magnitude and phase paths. The amplitude modulator 22 receives the amplitude data ρ(t) in the magnitude path and modulates a power supply voltage (Vsupply) according to the amplitude of ρ(t). The phase modulator 24 receives the constant-amplitude phase data dφ/dt in the phase path, and drives the voltage controlled oscillator (VCO) 26 to provide an RF drive signal to the RFPA 28.
The RFPA 28 in the polar modulator 2 in FIG. 2 is configured as a highly-efficient nonlinear switched-mode RFPA. The RFPA 28 remains in compression while the drain voltage of the RFPA 28 is varied. By modulating the drain supply, the amplitude information of non-constant envelope signals such as EDGE and W-CDMA can be efficiently superimposed on the RF signal from the phase path of the modulator 2.
The amplitude modulator 22 in the amplitude path of the polar modulator 2 may be formed in various ways. One known approach is to use a class-S amplitude modulator 30 to modulate the drain supply to the RFPA 32 of the polar modulator, as shown in the polar modulator 3 depicted in FIG. 3. Class-S modulators are more efficient than conventional linear modulators and, when configured for amplitude modulation in the magnitude path of a polar modulator, provide a highly-efficient means for modulating an RF carrier. Further, because the class-S modulator 30 is configured within the magnitude path of the polar modulator 3, and the drive signal to the RFPA 32 is a constant-magnitude phase modulated signal within the phase path of the polar modulator 3, distortion caused by the clipping of signal peaks in non-constant envelope signals, such as EDGE and W-CDMA, can be avoided.
A more detailed view of a class-S modulator 4 is shown in FIG. 4. The class-S modulator 4 comprises a comparator 40, a level shifter and gate driver 41, and a buck converter that includes a switching transistor 42, a diode 43, an inductor 44 and a capacitor 45. When configured in the amplitude path of a polar modulator, the amplitude values in the amplitude path are compared to a triangular reference signal to produce a pulse-width-modulated (PWM) signal. The resulting PWM signal is comprised of a series of pulses having durations that vary in proportion to the amplitude values at the comparator input.
Together, the switching transistor 42 and diode 43 in the buck converter portion of the class-S modulator act as a single-pole double-throw (SPDT) switch. Current flows through switching transistor 42 when it is ON and through diode 43 when transistor 42 is OFF. Switching the buck converter with the PWM signal from the driver generates a high-level PWM signal. The high-level PWM signal is converted back into an analog signal by the low-pass output filter formed by the inductor 44 and the capacitor 45.
Accurate envelope tracking of the amplitude signal requires that the switching frequency of the buck converter be about twenty to fifty times higher than the required signal envelope bandwidth. For a signal such as EDGE, the envelope bandwidth is approximately 1 MHz. This means that for EDGE type signals, the transistor in the buck converter would have to be capable of switching at a rate of 20-50 MHz rate. Unfortunately, the switching transistor (typically a silicon-based MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) or BJT (Bipolar Junction Transistor)) in state-of-the-art buck converters can only be switched up to a maximum of about 5 MHz. For this reason, conventional class-S modulators are not well-suited for EDGE and other non-constant envelope signals that have high signal envelope bandwidths.
What is needed, therefore, is a modulator circuit for a polar modulator that has an efficiency similar to or better than a conventional class-S modulator, and which is capable of transmitting and conditioning non-constant envelope signals such as EDGE and W-CDMA, while satisfying power spectra requirements specified by these standards.